To adjust the flow rate distribution so that the thickness of a sheet is uniformized in the width direction of a die for the flowing characteristics of resin in the wide range by adjusting the movement amount of a clearance adjusting body for adjusting clearances of a gradually enlarging passage and a slits.

Resin is flowed from a resin inlet 4 into a manifold 5, passed through an inclined passage 14, a slit 15 and a slit 6, while being expanded in the width direction and extruded put of a lower outlet 6a. As the flow path resistance of a resin passage passing through a die central section and the resin passage passing through both ends of the die are designed to be almost equal by setting the inclined passage 14 and the inclined slit 15, the thickness of a sheet to be extruded is almost uniform all over the whole width. In the case of using resins of different flowing characteristics, a difference is generated in the flow path resistance on the die central section and both end sections to vary the thickness of the sheet, and the position of a clearance adjusting body 16 is moved and the clearances are adjusted so that the flow path resistance is uniformized in the width direction of the die.